In recent years, devices based on a method such as MPEG (Moving Picture Experts Group phase) for performing compression with orthogonal transform such as discrete cosine transform and motion compensation by making use of redundancy unique to image information are widely spreading in not only information distribution such as a broadcast station but also information reception for ordinary households.
In particular, the MPEG2 (ISO/IEC 13818-2) method is defined as a general-purpose image coding method, and is a standard that covers both of interlaced scanning images and progressive scanning images, and standard resolution images and high resolution images. For example, MPEG2 is now widely used for wide range of applications such as professional use and consumer use. For example, in a case of an interlaced scanning image of a standard resolution of 720 by 480 pixels, the amount of codes (bit rate) of 4 to 8 Mbps is allocated. For example, in a case of an interlaced scanning image of a high resolution of 1920 by 1088 pixels, the amount of codes (bit rate) of 18 to 22 Mbps is allocated. Accordingly, a high compression rate and high image quality can be achieved by using the MPEG2 method.
MPEG2 is mainly targeted for high image quality coding suitable for broadcasting, but does not support coding method using less amount of codes (lower bit rate) than MPEG1. In other words, MPEG2 does not support a coding method of a higher compression rate. As portable terminals become widely prevalent, needs for such coding methods are considered to grow in the future, and in order to respond to such needs, MPEG4 coding method has been standardized. With regard to image coding method of MPEG4, the specification is admitted as ISO/IEC 14496-2 M international standard on December 1998.
Further, in recent years, a standard called H. 26L (ITU-T Q6/16 VCEG) has been standardized which was at first for the purpose of image coding of television conference. As compared with coding methods such as MPEG2 and MPEG4, H. 26L requires a higher amount of computation because of encoding and decoding process, but is known to achieve a higher encoding efficiency.
In recent Years, as a part of activity of MPEG4, Joint Model of Enhanced-Compression Video Coding has made standardization for achieving a higher encoding efficiency which is based on H. 26L and further employs functions not supported in H. 26L. This standardization has made into an international standard under the name of H. 264 and MPEG-4 Part 10 (AVC (Advanced Video Coding)) in March 2003.
Further, as an expansion thereof, FRExt (Fidelity Range Extension) including an encoding tool required for works on color difference signal formats such as 4:2:2 and 4:4:4, and RGB, and including 8 by 8 DCT (Discrete Cosine Transform) and quantization matrix defined in MPEG2, was standardized in February, 2005. Therefore, the AVC method is made into a coding method capable of finely expressing even film noises included in movies, and has made progress to be used in a wide range of applications such as BD (Blu-ray (registered trademark) Disc).
However, recently, the needs for a still higher compression rate encoding process, such as compression of an image having about 4000 by 2000 pixels which is about four times the high-definition television image or distributing a high-definition television image in an environment where there is only a limited transmission capacity such as the Internet, is growing. Therefore, VCEG (Video Coding Expert Group) under the ITU-T is continuing to study the improvement of the encoding efficiency.
Currently, in order to improve the encoding efficiency still greater than the AVC, JCT-VC (Joint Collaboration Team-Video Coding) which is a joint standardization organization of ITU-T and ISO/IEC has been standardizing an encoding method called HEVC (High Efficiency Video Coding). As of October 2013, NPL 1 has been issued as Draft.
However, where the size of TU (transform unit) is 4 by 4 pixels, the HEVC can use a function of transform skip in which orthogonal transform and inverse-orthogonal transform are not performed on the TU in question.
More specifically, when an image which is to be encoded is a nonnatural image such as a CG (Computer Graphics) image and a screen of a personal computer, 4 by 4 pixels is likely to be selected as the size of the TU. The encoding efficiency may increase when the orthogonal transform is not applied to nonnatural images. Therefore, when the size of TU is 4 by 4 pixels, the HEVC improves the encoding efficiency by allowing the transform skip to be applied.
On the other hand, in NPL 2, an encoding method is considered to improve images in color difference signal formats such as 4:2:2 and 4:4:4 and encoding of screen contents.
NPL 3, encoding efficiency is considered where transform skip is applied a TU having a size larger than 4 by 4 pixels.
Further, in NPL 4, when the minimum size of the TU is, for example, 8 by 8 pixels, which is other than 4 by 4 pixels, the transform skip is considered to be applied to the TU of the minimum size.